1. Field of the Invention
The present invention relates to liquid fuel feed systems, and more particularly to charge forming devices which compare the flow rate of the air and the fuel in order to control the fuel delivered by the system
2. History of the Prior Art
The conventional gasoline carburetor has a fuel chamber which is vented to the outside air. A fuel inlet needle valve is controlled by an adjustable float to provide the proper liquid level of fuel within the fuel chamber. The movement of air through a venturi bore provides the pressure difference necessary to move the fuel into the air stream.
The conventional charge forming device senses a quantity of air movement through the venturi by means of an air sensing diaphragm, and balances the force on the air sensing diaphragm against that of an opposing force by a fuel-sensing diaphragm, which senses the quantity of fuel movement across a fuel orifice. The two diaphragms are connected by a member which controls a fuel discharge valve. An increase or decrease in air movement unbalances the diaphragms and causes a repositioning movement of the diaphragms to a balanced position. An increase in air movement causes a diaphragm movement which further opens a fuel discharge valve, and a decrease in air movement causes a diaphragm movement which restricts the fuel discharge valve. However, in order to assure a proper liquid head of fuel to the fuel discharge valve, prior art charge forming devices must rely upon a constant fuel inlet pressure such as provided by a rotary vane type of fuel pump.
Also, conventional carburetors and charge forming devices require a sufficient movement of air through the venturi bore before fuel will flow to the engine. At cranking speeds, it has been necessary to utilize choke plates to increase the pressure difference across the venturi in order to allow fuel to flow, or to provide manual primers to pump fuel into the inlet bore which allows the engine to start. This has been especially true for small one or two cylinder air-cooled engines which have relatively slower cranking speeds. As a result, certain small engines have been eliminated from being utilized in applications requiring remote or automatic starting, such as backup generators or pumps.
Another problem with the conventional carburetors and charge forming devices is the drift in the air density which alters the air-fuel ratio, which is a mass relationship rather than a volumetric relationship. When a carburetion system is sized for a particular cubic inch displacement engine, the cross-sectional area of the venturi bore is sized for a flow rate which is based upon the air density at a given altitude and temperature. An increase in altitude and/or temperature results in a lighter air density which causes the base air-fuel mixture to drift to a rich ratio. Such ratios result in higher proportions of unburned pollutants released to the atmosphere and in a reduced fuel economy. A decrease in altitude and/or temperature results in a lean ratio which results in higher emissions, a loss of engine power due to a slower combustion, and also a reduced fuel economy. For example, a base ratio of 12.8 to 1 at sea level and 60.degree. F., will drift to lean mixture of 14.76 to 1 at 1000 feet below sea level and -40.degree. F., and will drift to a rich mixture of 9.5 to 1 at 12,000 feet above sea level and 140.degree. F. The drift at sea level within a temperature range of -40.degree. F. to 140.degree. F. is from a ratio of 14.23 to 1, to a ratio of 11.91 to 1.
It would be an advantage to have a charge-forming device which could internally control the liquid head of the fuel to the discharge valve; which could provide for remote and automatic starting capability; and which could provide a stabilized air-fuel ratio for various ranges of air densities.